The consensus among the pundits after last night's debate seemed to be that the presidential election belongs to Obama, unless something really wacky and unforeseen occurs. Voters at this point are preferring Obama on the economy, and a huge majority of Americans say that the economy is the most important issue. So unless something more beneficial to McCain becomes the hot issue (Russians invading Alaska, held at bay by lipstick-wearing hockey moms?), it's hard to see McCain winning.
That said, Obama may well be at or near the ceiling of his potential support level. The trend lines at Pollster seem to be showing both candidates' support flattening out, with Obama holding just above 49 and McCain just above 43. Each candidate has gained about 0.1 percentage points for each of the past few days, suggesting that undecideds are, very slowly, making up their minds and splitting between the two candidates. As Nate Silver at 538 points out, for Obama to get much more, he'll have to start picking up the racist vote, and that's just not likely to happen. Meanwhile, in an open-seat presidential race, the loser almost never goes below 45% of the vote, suggesting that McCain isn't likely to drop any lower than where he is today.
On top of that, elections tend to tighten up a bit in the final few weeks. That's not a great cause for Obama supporters to worry -- he doesn't have to pick up that many of the undecideds to put him over the top -- but it is something to keep in mind. The media will grow tired of reporting the status quo for another four weeks. Soon the story will be McCain's modest surge. Is it Palin? Is it McCain? Is it the new thuggish campaign? Don't let it throw you; it's typical in an election.
Right now I'd say the campaign dynamics are on target for a roughly 5-point Obama win. We'll see....